1. Field of the Invention:
The present invention relates to the preparation of N-(dichlorophosphoryl)trichlorophosphazene and, more especially, to the preparation of N-(dichlorophosphoryl)trichlorophosphazene that is essentially pure and colorless.
2. Description of the Prior Art:
U.S. Pat. No. 3,231,327 describes the preparation of N-(dichlorophosphoryl)trichlorophosphazene having the formula: ##STR1## or, more simply, P.sub.2 NOCl.sub.5.
According to this '327 patent, P.sub.3 NCl.sub.12 is initially prepared by the following simplified reaction scheme: EQU 3PCl.sub.5 +NH.sub.4 Cl.fwdarw.(Cl.sub.3 P--N--PCl.sub.3).sup.+ PCl.sub.6.sup.- +4HCl
and the P.sub.3 NCl.sub.12 is then decomposed to P.sub.2 NOCl.sub.5 under the action of P.sub.2 O.sub.5.
In French application No. 86/15684, it is proposed to treat the reaction medium containing P.sub.3 NCl.sub.12 with SO.sub.2 The described method comprises the decomposition of P.sub.3 NCl.sub.12 with SO.sub.2 at a low temperature (at most 30.degree. C.), in order to provide, after distillation, a final product (P.sub.2 NOCl.sub.5) devoid of yellow discoloration, as the impurities which impart such discoloration interfere with the subsequent polycondensation of P.sub.2 NOCl.sub.5 to polychlorophosphazene.
This particular operation, which is carried out at a low temperature to prevent discoloration due to the presence of SO.sub.2, results in the evaporation of SOCl.sub.2 and POCl.sub.3 while maintaining the same temperature. Such low temperature evaporation makes it necessary to conduct the operation under a high vacuum (pressure reduced to 1 torr, for example), and to condense the vapors of SOCl.sub.2 and POCl.sub.3 at a very low temperature, on the order of -80.degree. C.